Messages in this thread | | | Date | Fri, 08 Feb 2013 22:16:47 -0500 (EST) | From | Nicolas Pitre <> | Subject | Re: [RFC] arm: use built-in byte swap function |
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On Fri, 8 Feb 2013, Kim Phillips wrote:
> On Fri, 8 Feb 2013 17:47:33 -0500 > Nicolas Pitre <nico@fluxnic.net> wrote: > > > On Fri, 8 Feb 2013, Woodhouse, David wrote: > > > > > On Fri, 2013-02-08 at 15:04 -0500, Nicolas Pitre wrote: > > > > On Fri, 8 Feb 2013, Woodhouse, David wrote: > > > > > > > > > On Thu, 2013-02-07 at 18:13 +0000, Russell King - ARM Linux wrote: > > > > > > > > > > > > However, the biggest reason not to use libgcc is that we want to control > > > > > > what gets used in the kernel - for example, no floating point, and no > > > > > > use of 64 x 64bit division. > > > > > > > > > > Which is all very sensible. But there's no particular reason we couldn't > > > > > add a __bswap[sd]i2 to the kernel's version of libgcc if we wanted to. > > > > > > > > Absolutely. > > > > > > And then ARM can just set ARCH_USE_BUILTIN_BSWAP like other > > > architectures do, right? > > > > If that turns out to be beneficial over what we have now, then yes. > > I didn't read back the whole thread to form an opinion though. > > The diff below implements __bswap[sd]i2 in arch/arm/lib, and > results in the following savings in vmlinux size: > > column 1: name of defconfig > column 2: text+data+bss, linux-next-20130207 vanilla, gcc 4.6.3 > column 3: text+data+bss, linux-next-20130207+below diff, gcc 4.6.3 > column 4: col. 3 - col. 2 (ie., -ve numbers represent savings) > [...] > imx_v6_v7_defconfig: 7672373 7667089 -5284 > lart_defconfig: 2941150 2941054 -96 > mxs_defconfig: 11091983 11095679 3696
The savings are good, with some impressive cases. However the mxs_defconfig is completely the opposite and by far. Any idea?
> gcc 4.7.3 runs haven't been as good across the board as gcc 4.6.3, > however:
Not only that, but in many cases the results are wildly different given the same config:
> imx_v6_v7_defconfig: 7637605 7636935 -670 > lart_defconfig: 2922550 2926600 4050 > mxs_defconfig: 11071139 11070893 -246
The mxs_defconfig became much better while lart_defconfig regressed a lot.
> Haven't looked at why.
Would be a good idea since this is rather weird and gcc could benefit from your findings.
> In any case, some questions I have are: > > (a) are the __bswap[sd]i2 implementations acceptable written in C, > as in the diff? I don't speak ARM asm (yet at least). The > generated code looks pretty optimal in both armv5 and 6+.
It looks pretty nice indeed:
__bswapsi2: eor r2, r0, r0, ror #16 mov r1, r2, lsr #8 bic r3, r1, #65280 eor r0, r3, r0, ror #8 bx lr
There is no way to do better than that. But that's true only if -Os is _not_ used. With -Os we get the following output:
__bswapsi2: mov r3, r0, asl #24 and r2, r0, #65280 orr r3, r3, r0, lsr #24 orr r3, r3, r2, asl #8 and r0, r0, #16711680 orr r0, r3, r0, lsr #8 bx lr
I really don't get why gcc thinks the above is shorter. I'm saving you from pasting the __bswapdi2 result which is also way way worse. That was with Linaro gcc v4.6.2.
With Sourcery gcc v4.5.1 we get:
__bswapsi2: stmfd sp!, {r3, lr} bl __bswapsi2 ldmfd sp!, {r3, pc}
This is indeed shorter, but much less useful. So you better enforce -O2 for this file. And the nice thing with C code is that it is fully optimized with the rev instruction when compiling for ARMv6+ if it is ever used in that case.
> (b) would adding __bswap[sd]i2 to the kernel build need to be > disabled on armv6+? AFAICT, gcc doesn't emit calls - even for the > 8-byte swap, even with -Os, on armv6+.
I wouldn't bother. That would save only 6 instructions total. And who knows if some gcc flavor start calling them for some reason eventually.
> (c) testing allyesconfigs is proving to be a pain - lots of > breakeage - other than defconfig testing, is there any more I can do?
The defconfigs provide wildly different results and that is a good thing for further investigation. You may concentrate on a small interesting sample such as those I kept above.
With allyesconfig the good configs would cancel out the bad ones making the bad ones invisible.
Nicolas
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